As a method of generating a continuous terahertz wave, OE conversion (Optical to Electrical conversion) by a photoconductive device or photodiode has recently received attention because of a wide frequency range. A frequency multiplier using a nonlinear diode element has been employed conventionally for a high frequency region (>300 GHz) in which a transistor amplifier cannot reach. However, the frequency multiplier is not always suited to a wide variety of applications because, for example, the bandwidth is generally narrow and the signal modulation device configuration is complicated.
If a beat signal of two optical modes, in which two laser beam outputs (light wavelengths: λ1 and λ2) are combined by making their polarizations match each other, are OE-converted, a terahertz wave can be easily extracted as a difference frequency between them. Since the optical beat signal can be used, the usable wavelength range is very wide, as described above.
However, a maximum terahertz wave output which can be generated by one OE conversion device is generally limited owing to restrictions accompanying the frequency characteristic of a device and heat generation. For this reason, a technique of generating a higher terahertz wave output by power combining is desired especially in an application of signal transmission. It is also fundamentally important to increase an output from a terahertz wave light source, in order to shorten the observation time of terahertz wave imaging. The power combining includes a method of combining terahertz wave outputs from respective OE conversion devices on an electrical line, and a method of directly connecting antennas to OE conversion devices. In the latter method, a so-called phased array antenna can be configured, so various functions can be given by controlling the radiation pattern.
Efficient detection of terahertz wave imaging becomes possible by supplying local signals to many OE conversion devices even in homodyne detection and heterodyne detection using the nonlinearity of the OE conversion devices. Also, a wireless system using a phased array antenna can be built.
In any case, when a terahertz wave signal is generated or received using many OE conversion devices, a technique of distributing a controlled optical beat signal to the respective OE conversion devices is fundamentally important.
Generally in terahertz wave power combining, an optical signal distribution circuit 100 is used to supply a plurality of branched optical beat signals to a plurality of OE conversion devices and combine generated terahertz waves. As shown in FIG. 6, an optical beat signal input obtained by multiplexing a λ1 signal and λ2 signal is coupled to an input waveguide port 101, and then branched into four by series-connected 1×2 optical splitters 104, 105, and 106 in two stages on connection waveguides 102.
After that, optical beat signals output from the 1×2 optical splitters 105 and 106 via output waveguides 103 are coupled to photodiodes 108 via an optical lens array 107. Terahertz wave outputs from the photodiode 108 are power-combined by a circuit on electrical lines 109. It is easy at the accuracy of terahertz wavelength level to equalize waveguide lengths from the optical input port to output port of a PLC (Planar Lightwave Circuit). The phases of terahertz wave outputs from the photodiodes can be controlled to be uniform.
The photodiodes and power combining circuit can be integrated on a single substrate 110. The combined terahertz wave power is coupled to, e.g., a metal waveguide and emitted finally to the space. In addition to the power combining using the electrical line as shown in FIG. 6, terahertz wave outputs from respective photodiodes can be directly coupled to one of many arrayed antennas to combine powers as an array antenna output. An optical signal distribution circuit similar to one in FIG. 6 can be used for an application in which the phases of terahertz waves output from the respective antennas are fixed.